The present invention relates to a hydraulic tappet designed for assembly on a drive for controlling an internal combustion engine valve.
Hydraulic tappets of the aforementioned type are generally known to comprise a first member, sliding in relation to the engine frame, and a second member sliding axially in relation to the said first member. Between the said two members is formed a chamber varying in volume and having and inlet duct for activating fluid, the opening on the said duct being controlled by an on-off member in such a manner that the said fluid flowing into the said chamber causes one of the said members to move axially in relation to the other so as to vary the volume of the said chamber and so take up any slack on the said drive gear train.
The said variable-volume chamber normally communicates through ducts with an appropriate source of activating fluid from the engine.
A major drawback on tappets of the aforementioned type is that they perform badly when the engine is started up, owing to the fact that the oil inside the said chamber prior to start-up seeps out between the mating surfaces on the said first and second sliding members, and that, during start-up, the oil pressure on the engine is too low to ensure adequate oil supply to the said chamber. On certain known tappets, the aforementioned drawbacks have been partially solved by providing for a second activating fluid chamber communicating hydraulically with the first and designed to store a certain amount of fluid even when the engine is idle, thus enabling the first chamber to be filled with fluid more easily, as compared with the previous arrangement, when the engine is started up.
On these tappets, however, long-term operation of the engine may result in such severe oil leakage from the second chamber as to jeopardize operation of the tappet; such leakage occurring between the mutually-sliding mating surfaces on the said two sliding members, despite the same being provided with appropriate sealing members.